A gradient photochromic glass article is a glass article exhibiting a gradient in photochromic properties thereacross. One example is a photochromic glass article having a first region exhibiting properties normal for a photochromic glass and a second region exhibiting photochromic properties which are at least substantially less pronounced than those of the first region. Often the second region is non-photochromic. The transition from the strongly photochromic to the weakly photochromic or non-photochromic region may be gradual or abrupt.
A number of different methods for producing gradient photochromic glass articles are known. U.S. Pat. No. 3,419,370 to Cramer et al., for example, describes an ionexchange method for producing photochromic surface layers on glass which can be utilized to provide photochromic properties in selected regions of a glass article. Canadian Pat. No. 739,404 to Brown et al. describes a method wherein potentially photochromic glass is heat treated in a partitioned lehr, or while partly encased in a refractory block, so that only part of the glass is heated sufficiently to become photochromic. In this method, selective heating or, equivalently, selective insulation from heat are utilized to control photochromic development.
German Patentschrift No. 2,125,232 describes a radiant heating assembly which can be adapted to the production of gradient photochromic glass by selective heating. U.S. Pat. No. 4,036,624 illustrates the use of a gradient furnace to produce gradient photochromic ophthalmic lenses.
It is evident from a study of the prior art that past methods of producing gradient photochromic glass articles through heat treatment involve the suppression of photochromic development in selected portions of the articles by protecting those portions from elevated temperatures. Hence the nucleation and growth of photochromic phases in the glass is retarded by keeping selected portions thereof relatively cool.
For some applications it may be desirable to heat treat potentially photochromic glass at temperatures near its softening point to simultaneously achieve the development of photochromic properties and the sagging of the glass to a desired configuration. This can not be accomplished using prior art photochromic gradient heat treating methods because the temperature gradient which must be maintained in the glass results in non-uniform sagging.
It is a principal object of the present invention to provide a process for producing gradient photochromic glass articles which avoids this and other disadvantages of prior art treatments, offering improved flexibility with respect to the step of heat treating the glass for photochromic development.
Other objects and advantages of the invention will become apparent from the following description and examples thereof.